This invention relates to a CRT (Cathode Ray Tube) frit and, in particular, to a CRT frit for use in sealing a panel and a funnel of a color CRT bulb.
Traditionally, a panel and a funnel of a color CRT bulb are sealed by the use of a crystallizing CRT frit comprising crystallizing glass powder of PbOxe2x80x94B2O3xe2x80x94ZnOxe2x80x94SiO2xe2x80x94BaO and refractory filler powder such as zircon. In a sealing step, the crystallizing CRT frit is held at a temperature between 440 and 460xc2x0 C. for 30 to 60 minutes.
During an exhaust step following the sealing step, the bulb is heated again to a temperature between 350 and 400xc2x0 C. Since the crystallizing CRT frit is excellent in heat resistance, no displacement of the panel and the funnel is caused in the exhaust step as a result of softening of the crystallizing CRT frit.
Following the recent improvement in the exhaust technique, it is possible to carry out exhaust by low-temperature heating. Therefore, the CRT frit is not required to have such a high heat resistance. Instead, in view of the reduction in energy cost and the improvement in productivity, it becomes important to seal the bulb at a lower temperature and in a shorter time. However, the existing CRT frit can not meet such a demand because high-temperature and long-time sealing is required as described above in order to obtain desired characteristics.
It is therefore an object of this invention to provide a CRT frit capable of sealing a panel and a funnel of a CRT bulb at a low temperature and in a short time.
According to this invention, there is provided a CRT frit which is for use in sealing a panel and a funnel of a CRT bulb and which comprises non-crystallizing glass powder and refractory filler powder, the non-crystallizing glass powder consisting essentially of, by weight percent, 75-90% PbO, 7-20% B2O3, 0-8% ZnO, 0-5% SiO2, and 0.1-8% Al2O3+Fe2O3.
Now, description will be made about this invention.
A CRT frit according to this invention comprises powder of a non-crystallizing glass and powder of a refractory filler.
The non-crystallizing glass used in this invention is highly stable and is hardly devitrified. The non-crystallizing glass has a glass transition point on the order between 290 and 310xc2x0 C. and a softening point on the order between 330 and 370xc2x0 C. The non-crystallizing glass is excellent in flowability because it is not crystallizable. Therefore, with the CRT frit of this invention, it is possible to seal the bulb at a low temperature and in a short time. Preferably, the non-crystallizing glass consists essentially of, by weight percent, 76-88% PbO, 8-18% B2O3, 0.1-5% ZnO, 0.1-4% SiO2, and 0.1-7% Al2O3+Fe2O3.
The non-crystallizing glass is classified into a first glass composition which essentially contains Al2O3 and a second glass composition which essentially contains Fe2O3.
Specifically, the first glass composition consists essentially of, by weight percent, 76-90% PbO, 8-18% B2O3, 0-5% ZnO, 0.1-3% SiO2, and 0.1-3% Al2O3. The first glass composition tends to provide the glass having a relatively high softening point, as compared with the second glass composition. Therefore, displacement of the panel and the funnel resulting from softening of the glass and generation of bubbles from the glass hardly occur. In case where the exhaust temperature of the CRT bulb in an exhaust step can not sufficiently be lowered, the first glass composition is advantageous because of high stability. Description will hereinafter be made about the reason why the first glass composition is determined as mentioned above.
PbO is a component forming a network structure of the glass The content of PbO is 76-90%, preferably, 79-87%. If the content of PbO exceeds 90%, the coefficient of thermal expansion becomes too great. If the content is less than 76%, the flowability of the glass is lowered and the sealing temperature becomes excessively high.
B2O3 is a component forming the network structure of the glass. The content of B2O3 is 8-18%, preferably, 10-15%. If the content of B2O3 exceeds 18%, chemical durability is degraded to a practically unfavorable level. If the content is less then 8%, the flowability of the glass is decreased.
ZnO serves to suppress devitrification of the glass if it is added in a predetermined amount. The content is 0-5%, preferably, 1-4%. If the content of ZnO exceeds the above-mentioned range, devitrification will readily occur.
SiO2 serves to stabilize the glass of the above-mentioned composition. The content is 0.1-3%, preferably, 0.2-2.5%. If the content of SiO2 is more than 3%, the softening point is elevated and the sealing temperature becomes excessively high. If the content is less than 0.1%, the glass becomes unstable and will be devitrified.
Al2O3 serves to stabilize the glass of the above-mentioned composition. The content is 0.1-3%, preferably, 0.2-2.5%. If the content of Al2O3 is more than 3%, the softening point is elevated and the sealing temperature becomes excessively high. If the content is less than 0.1%, the glass becomes unstable and will be devitrified.
Furthermore, V2O5 or Bi2O3 may be contained although V2O5 or Bi2O3 is not essential. In case where the softening point is excessively elevated as a result of addition of SiO2 and Al2O3 as the glass stabilizing components, addition of V2O5 or Bi2O3 is desired to control the softening point.
V2O5 serves to lower the softening point of the glass of the above-mentioned composition. The content is 0-1%, preferably, 0-0.8%. If the content of V2O5 is more than 1 %, devitrification will readily occur.
Bi2O3 is a component which serves to lower the softening point of the glass of the above-mentioned composition. The content is 0-5%, preferably, 0-4%. If the content of Bi2O3 is more than 5%, devitrification will readily occur.
As far as the glass does not become unstable, other components can be added. For example, in order to prevent the release of PbO, TiO2 may be added up to 5%. In case where the glass is unstable and tends to be devitrified, Fe2O3 or CuO may be added to stabilize the glass. The content of Fe2O3 is 5% or less, preferably, 2% or less. The content of CuO is 3% or less, preferably, 1% or less.
On the other hand, the second glass composition consists essentially of, by weight percent, 75-90% PbO, 7-20% B2O3, 0-8% ZnO, 0-5% SiO2, and 0.1-5% Fe2O3. Since the second glass composition contains Fe2O3, the glass is highly stable and devitrification (surface crystal precipitation) upon sealing will very hardly occur. As compared with the first glass composition, the second glass composition tends to provides the glass having a relatively low softening point. Therefore, in order to further lower the sealing temperature, the second glass composition is advantageous. Description will hereinafter be made about the reason why the second glass composition is determined as mentioned above.
PbO is a component forming a network structure of the glass. The content of PbO is 75-90%, preferably, 79-87%. If the content of PbO exceeds 90%, the coefficient of thermal expansion becomes too great. If the content is less than 75%, the flowability of the glass is lowered and the sealing temperature becomes excessively high.
B2O3 is a component forming the network structure of the glass. The content of B2O3 is 7-20%, preferably, 9-15%. If the content of B203 exceeds 20%, chemical durability is degraded to a practically unfavorable level. If the content is less then 7%, the flowability of the glass is degraded.
ZnO serves to suppress devitrification of the glass if it is added in a predetermined amount. The content is 0-8%, preferably, 1-5%. If the content of ZnO exceeds the above-mentioned range, devitrification will readily occur.
SiO2 serves to stabilize the glass of the above-mentioned composition. The content is 0-5%, preferably, 0.2-2.5%. If the content of SiO2 is more than 5%, the softening point is elevated and the sealing temperature becomes excessively high.
Fe2O3 is a component for stabilizing the glass to suppress devitrification upon sealing. In addition, Fe2O3 serves to improve weather resistance of the glass so as to prevent degradation in quality during storage, to enhance infrared absorption of the glass so as to facilitate softening and flowing of the glass, and to reduce the amount of PbO released out of the glass. The content of Fe2O3 is 0.1-5%, preferably, 0.3-4%. If the content of Fe2O3 is more than 5%, the softening point is elevated and the sealing temperature becomes excessively high. If the content is less than 0.1%, the above-mentioned effects can not be obtained. For example, the glass becomes unstable to be devitrified, or deteriorated during storage to be degraded in flowability.
In order to further stabilize the glass, Al2O3 or Bi2O3 may be contained.
The content of Al2O3 is 0-5%, preferably, 0-2.5%. If the content of Al2O3 is more than 5%, the softening point is elevated and the sealing temperature becomes excessively high.
The content of Bi2O3 is 0-6%, preferably, 0-3%. If the content of Bi2O3 is more than 6%, devitrification will readily occur.
As far as the glass does not become unstable, other components can be added. For example, in order to prevent the release of PbO, TiO2 can be added up to 5%. In case where the glass is unstable and tends to be devitrified, CuO or GeO2 may be added to stabilize the glass. The content of CuO is 3% or less, preferably, 1% or less. The content of GeO2 is 5% or less, preferably, 2% or less. In order to lower the temperature, Cs2O and Ag2O may be added up to 1% and up to 0.8%, respectively. If the content of Cs2O exceeds 1%, the glass is deteriorated during storage to degrade the flowability. If the content of Ag2O exceeds 0.8%, the production cost is unfavorably increased.
As the refractory filler powder in this invention, use of alumina (Al2O3) or zircon (ZrSiO4) is most preferable. Besides, use may be made of cordierite (2MgO.2Al2O3.5SiO2), lead titanate (PbTiO3), silica glass (a-SiO2), willemite (2ZnO.SiO2), and tin oxide (SnO2). These materials may be used alone or in combination.
In this invention, the ratio of the non-crystallizing glass powder and the refractory filler powder is between 95:5 and 55:45, preferably, between 91:9 and 70:30 in weight ratio. The reason why the ratio of the non-crystallizing glass powder and the refractory filler powder determined as mentioned above is as follows. If the content of the glass powder is smaller than the above-mentioned range, the flowability of the frit is insufficient. In this event, it is impossible to obtain an excellent sealing configuration and to form a compact sealing layer. On the other hand, if the content of the refractory filler powder is smaller than the above-mentioned range, the frit and the bulb does not match in coefficient of thermal expansion and the mechanical strength is insufficient.
In the CRT frit of this invention, the coefficient of thermal expansion at a temperature between 30 and 250xc2x0 C. is preferably adjusted to a range between 80xc3x9710xe2x88x927/xc2x0 C. and 90xc3x9710xe2x88x927/xc2x0 C. If the coefficient of thermal expansion of the frit is within the above-mentioned range, stress of an appropriate level (450-1000 psi) is produced in the CRT bulb so that a high sealing strength is achieved. Beyond the above-mentioned range, abnormal stress is produced so that a frit seal portion, the panel, and the funnel will readily be damaged.
Since the CRT frit of this invention having the above-mentioned structure is not crystallized, it is unnecessary to consider crystallization characteristics upon sealing. Therefore, the sealing condition of the CRT bulb can be selected with a relatively large degree of freedom. Specifically, the sealing temperature is appropriately selected between 410 and 430xc2x0 C. while a holding time is appropriately selected between 5 and 20 minutes. As a most preferable sealing condition, the sealing temperature is 420xc2x0 C. and the holding time is 10 minutes. Depending upon the size and the shape of the CRT panel, the bulb may easily be broken due to the stress produced during sealing. In order to avoid such stress, the rate of elevation or drop of the temperature is moderated.